1. Field of the Invention
The present invention pertains to the field of porosimetry, or the measurement of the porosity of substances. More particularly, the invention relates to a liquid intrusion method for determining the porosity characteristics of a substance using a non-wetting liquid other than mercury.
2. Description of the Prior Art
The determination of porosity characteristics by intrusion of liquid mercury is a well known method. The method is used to determine the quantity of oil reserves in porous rock, in the fabrication of mineral raw materials to make ceramic or refractory bodies, and in many applications where the porosity of a substance must be ascertained.
Mercury is universally used as the intrusion liquid in commercial porosimetry since it does not wet most solids. That is, there is a repulsion of the liquid from the surface of the solid. Also, for a wide variety of solids, the "contact angle" of mercury is in the relatively narrow range of 135.degree. to 142.degree., so it does not have to be measured for each sample. The contact angle is used with the pressure and the surface tension of the intrusion liquid to determine the size of the pores into which the liquid is intruding. These characteristics make mercury the most practical liquid for general characterization of the porosity and pore structure of porous materials, and the liquid of choice for virtually all porosimetry in the past.
The method of porosimetry used is the following: A sample of material is placed in a sample chamber, which is evacuated to eliminate the air from the pores of the sample. The sample is then immersed in a non-wetting liquid, almost always mercury, as noted above. The mercury is then forced into the pores of the sample under pressure. By gradually increasing the pressure of a pressurization fluid (usually alcohol or oil) on the mercury supply and measuring the decrease in volume of the mercury surrounding the sample, the porosity characteristics of the material can be determined. The volume of mercury is determined by using a column of mercury as a reservoir, in which the height of the column can be accurately measured. The column is called a "penetrometer".
Problems often arise from using porosimeters of the mercury intrusion type to measure very small pores. Mercury has a relatively high surface tension, as well as a large non-wetting contact angle with most solids. Thus, very high pressures are needed to force mercury into the fine pores that are present in many materials, and the sample is sometimes crushed as a result. Commercial porosimeters typically operate at pressures up to 60,000 psia, which will intrude mercury into pores as small as 35 Angstroms in diameter. The use of non-wetting liquids other than mercury in porosimetry is potentially advantageous in that lower pressures can be used, thereby reducing the risk of crushing samples during testing.
In addition to the information obtained as to pore structure, it is sometimes desirable to test a material with a non-wetting liquid other than mercury that may actually be involved in the application or end use of the material, to obtain direct information about the solid-liquid interaction of interest. For example, the method of the invention has been used in porosimetry studies of TFE(Teflon.TM.)-bonded fuel cell electrodes in which the intrusion liquid is aqueous KOH, the electrolyte actually used in the operation of the electrodes. In this case the use of aqueous KOH rather than mercury as the intrusion liquid reduces the pressures required by a factor of 13.
In principle, any non-wetting liquid could be used for porosimetry, provided that the penetration volume can be measured accurately. In practice, this is difficult. The extremely high pressures involved, especially in mercury porosimetry, preclude the use of visual penetrometers, since the glass used to contain the columns of fluid could burst. The more reliable and accurate penetrometers in current use are based on one of two measurement techniques: the mercury in the penetrometer column may be used as part of an electrical circuit in which an electrical characteristic (such as resistance, inductance or capacitance) varies with the mercury level; or through the use of a magnetic float on top of the mercury column, using means for detecting the position of the float magnetically (see K. Gupta, U.S. Pat. No. 4,203,317). These techniques work well with mercury because of its electrical properties, for the first method, or its high density, for the second. It is unlikely that any other liquid of potential interest for porosimetry would work with a penetrometer based on one of these prevalent methods. Thus, simply substituting aqueous KOH, for example, for mercury in a conventional porosimeter would not be possible. At the same time, it would not be economical for most users to have a special porosimeter reserved for use solely with a liquid other than mercury, even if such could be built.
It is thus an object of the invention to provide a method for non-mercury porosimetry which can be performed using conventional porosimeters.
It is a further object of the invention to provide a method for non-mercury porosimetry which can be performed using porosimeters designed for mercury intrusion, without losing the capability of performing mercury porosimetry using the same equipment.